1. Field of the Invention
The present invention relates to an image forming apparatus, and specifically relates to a drive device for a photosensitive member in an image forming apparatus.
2. Description of the Related Art
In driving the photosensitive member in an image forming apparatus such as a copying machine, printer and the like, it is desirable to eliminate as far as possible the variation of rotation of the photosensitive member which produces deterioration in the quality of the formed image.
The photosensitive member drive mechanism used in conventional image forming apparatuses such as copying machines and the like, for example, drives the rotating shaft of the photosensitive member via a DC motor and a reduction gear. In such drive mechanisms, a large flywheel is mounted on the shaft of the photosensitive member, such that the inertia of the flywheel suppresses irregular rotation of the photosensitive member caused by the rotation fluctuation of the DC motor, the eccentricity of the reduction gear, and the clearance of the meshing portion (gear mesh clearance) of the reduction gears.
Provision of a rotary encoder on the photosensitive member has been proposed to control the rotation of the DC motor by directly detecting irregular rotation of the photosensitive member as well as changes in the surface speed of the photosensitive member.
The photosensitive member has also been driven via direct connection of the photosensitive member shaft and a stepping motor to eliminate the cause of rotational fluctuation caused by the reduction gear. In this instance, a microstep drive is employed to reduce rotational fluctuation generated by the step rise of the stepping motor.
The greatest disadvantage associated with image formation in such image forming apparatuses is the frequency component of rotational fluctuation below 100 Hz, the effects of which are humanly perceptible in a produced image. For example, FIG. 7 shows the rotational variation generated in the drive mechanism which rotates a photosensitive member 101 by a DC motor 102 via a reduction gear 103 shown in FIG. 6. In the drive mechanism of FIG. 6, a rotating shaft 101a of the photosensitive member 101 engages an output shaft 103d of the reduction gear 103, and the mutual gear ratios of the reduction gears 103a, 103b, 103c are set as integer multiples to fix the rotational variation generated by the gear eccentricity and the gear mesh clearance. The DC motor 102 suppresses rotational variation by rotational control, and rotational variation is further suppressed by providing a flywheel 104 on the output shaft of the reduction gear 103.
Referring now to FIG. 7, it can be understood that the periodic component of rotational variation under 200 Hz is manifested by the resonance of the gear, gear mesh clearance, and eccentricity of the motor output shaft. Among these, the rotational variation caused by the gear mesh clearance and the eccentricity of the motor output shaft are under 100 Hz.
It is difficult to suppress the low frequency components of the rotational variation caused by the eccentricity and the clearance of the gear mesh using a flywheel and the like. These components also are difficult to suppress due to the delay response in the controls even when the rotation of the motor is controlled by direct detection of change in surface speed and rotational variation of the photosensitive member.
On the other hand, it is possible to eliminate rotational variation caused by the gear mesh clearance when the drive is accomplished via a direct connection between the photosensitive member and the stepping motor. Although the stepping motor generates a rapid rotational variation during the step rise, this variation can be reduced by using a microstep drive. A drive by direct connection between the photosensitive member and the stepping motor is therefore desirable from the perspective of reducing rotational variation in the low frequency range.
When a stepping motor is used, however, rotational variation is caused by the torque ripple generated at each step period, and this rotational variation cannot be suppressed by the microstep drive. Since this rotational variation generated at each step period becomes 338 Hz when a typical two-phase stepping motor with a step angle of 1.8.degree. rotates, for example 338 pps as a normal rotation, this rotational variation itself is not manifested as a rotation period irregularity at under 100 Hz. The problem is the harmonic content of this rotational variation. In normal oscillation, a harmonic content is generated at n times and 1/n times (n: a natural number) one oscillation component.
That is, the harmonic content of the 338 Hz rotational variation is manifested at 169 Hz at 1/2 component, 112.7 Hz at 1/3 component, 84.5 Hz at 1/4 component, 67.6 Hz at 1/5 component, and 56.3 Hz at 1/6 component. These harmonic contents of under 100 HZ still affect the image despite the decrease in amplitude of the base rotational variation component as the denominator increases.
Furthermore, when driving via direct connection between the photosensitive member and the stepping motor, it is necessary to reduce the stepping speed to the previously mentioned 338 pps. When the stepping speed is reduced in this way, rotational variation readily occurs, and the scale of the motor must be increased due to the reduced torque. Since there motor torque decreases when a microstep drive is used, the motor must be enlarged to obtain torque.